Allow testing of signal strength

- Let any process with NETWORK_SETTINGS register for signal strength
  wakeup.
- Allow agents registering test networks to assign them a signal
  strength.

Test: NetworkAgentTest
Bug: 139268426
Change-Id: Iebfeb9316bcbd8472459c517abb16f1f9d879871
Merged-In: I2b4b89be3e69f4853fd6978d2c8f5c8eb4271f21
(cherry picked from commit 5cc7b18f, aosp/1284585)